I have a window suction feeder. I was given it as a present and I never thought it would work.
I put it out and nothing.
And then I had the idea of setting up a makeshift table further down the garden with food on it. The birds went to that. I then kept moving it closer to the house a little each day. Eventually they were visiting the table right by my window.
But they still did not go on the feeder. I used some bricks to raise the table so it was level with the feeder and put food right by the feeder. Once they got it and realised there was food there they've been visiting ever since. I took the table away as it had served its purpose.
And other birds now see them and it encourages them to visit. I've since put other feeders on hanging basket brackets right by my window.
One time I had a blackbird on my windowsill looking at me through the glass and singing quietly. It was a beautiful experience, as if he was thanking me for providing him with food.